Graphene is a material with a single layer of graphite crystals, and has attracted interest as a material with very unique physical properties. Among such unique attractive physical properties of graphene are high electrical mobility, high heat conductivity, high strength, and high light transmissivity. Such unique physical properties should be useful for providing novel electronics devices and nanotechnology materials.
Chemical exfoliation (Patent Literature 1) represents a conventional graphene producing method.
As used herein, the terms “single-layer graphene” and “multilayer graphene” refer to graphite present as a single layer, and graphite present as from 2 to 10 layers, respectively. Graphite having greater numbers of layers will be referred to as “graphite”. When stated simply as “graphene”, it refers to a state in which the single-layer graphene and the multilayer graphene coexist.
In the chemical exfoliation method, graphite oxide production proceeds by oxidizing a graphite powder with potassium permanganate in concentrated sulfuric acid. The reaction product is then immersed in sulfuric acid, and hydrogen peroxide is added for reaction.
The graphite oxide after the oxidation is irradiated with ultrasonic waves to exfoliate in layer direction.
The exfoliated graphite oxide is dispersed in purified water. The graphite oxide solution is centrifuged, and the supernatant is removed. The supernatant contains single-layer graphene oxide and multilayer graphene oxide.
FIG. 11 represents an SPM image (scanning probe micrograph) of a multilayer graphene oxide obtained by using the conventional producing method described in Patent Literature 1. The graphene oxide 1001 has a thickness of about 0.8 nm at analysis points 21, 22, and 23, and contains multiple (2 to 3) layers of graphene oxide. The graphene oxide 1001 turns itself into a multilayer graphene upon a reduction process performed at 200° C. to 1500° C. for 1 to 48 hours in a reducing atmosphere or a vacuum atmosphere.
FIG. 12 is a diagram representing the thickness and the frequency of randomly selected 100 samples of multilayer graphene oxide obtained by using the conventional producing method described in Patent Literature 1.
About 50% of the multilayer graphene oxides obtained by using the method of Patent Literature 1 have a thickness of 1 nm or less. These oxides appear to include single-layer graphene oxides. However, separation into single-layer graphene, and two- and three-layer graphenes is difficult, and graphenes with the desired number of layers cannot be obtained even after a reduction.